A chemical sensor is a device which monitors the concentration of a given chemical species in a liquid or a gas. Chemical sensors are used, for example, to detect unsafe levels of poisonous or explosive gases in the work and home environments.
Typically, chemical sensors comprise a sensitive layer, which is sensitive to a particular chemical which is to be detected by the sensor, and a heater. The heater increases the temperature of the sensitive layer to increase the sensitivity and selectivity of the sensor.
Chemical sensors formed using hybrid technology, such as for example sensors formed on ceramic substrates, are well known. It is also known to fabricate a semiconductor chemical sensor wherein the sensor and heater are integrated onto a silicon substrate.
These known chemical sensors, however, have poor thermal isolation between the heater and substrate which results in significant power consumption.
In order to reduce the power consumption in the semiconductor chemical sensors, it has been proposed to micromachine the backside of the bulk silicon to form a thin membrane under the heater. The thickness of the membrane is in the order of 2 microns. Although the thin membrane reduces power consumption, the thin membrane is fragile and liable to break during fabrication resulting in a loss in yield. Moreover, in view of the fragility of the membrane, the sensors have to be manually rather than automatically handled, which is extremely time consuming and so increases the cost of manufacture.
U.S. Pat. No. 5,345,213 describes a method for forming a semiconductor chemical sensor wherein the membrane is formed by surface micromachining and etching the silicon underneath the membrane using a mixture of ethylenediamine-pyrocatechol-water-pyrazine (EDP). However, EDP is an extremely dangerous chemical which is believed to be a carcinogen. It would be preferable to avoid using such a chemical.
It is therefore desirable to provide an improved semiconductor sensor device and an improved method for reducing power consumption in a semiconductor sensor device which does not suffer from the above prior art problems.